1. Field of the Invention
The present invention relates to a method for folding and curving of a metallic plate, more particularly one, in which a press is used, and a metallic plate is first pressed to have two parallel folded portions each having an obtuse angle and together defining an intermediate portion whose width is equal to the arc length of a trench of a lower die; thus, the intermediate portion will be curved to the same shape as the trench after it is sandwiched between and pressed with the lower die and an upper die fitting the trench, and in turns, two opposing straight portions adjacent to the curved intermediate portion will be parallel.
2. Brief Description of the Prior Art
Referring to FIG. 2, a metallic plate 2 is folded and curved by means of a press so as to have a folded portion 21 on a first edge, and a curved portion 22 adjacent to the folded portion 21. And, the plate 2 is cut to have another curved portion 23 at one end of a second edge thereof, which is adjacent to the first edge. And, the plate 2 is folded along the second edge such that the second edge touches the first one, as shown in FIG. 3. Furthermore, the edges are connected together by means of welding such that the metallic plate 2 has pleasant looking edges, and is suitable for use as a desk.
Referring to FIG. 4, a conventional press is used for folding and curving a metallic plate 2, which includes first and second lower dies 3 and 5, and hydraulic actuated upper dies 4 and 6. The lower die 3 is elongate, and formed with a notch 31 along it while the lower die 5 is elongate, and formed with first and second trenches 51 and 52 along it, whose cross-sections are substantially like an U having a semi-circular bottom or a semi-oval one; the trench 51 is shallower than the trench 52. The upper die 4 is elongate, and formed with a sharp edge along a lower end while the upper die 6 is elongate, and formed with a pressing portion along a lower end and a recess 61 along a lateral side, which pressing portion has such a shape as to fit the trenches 51 and 52 of the second lower die 5.
The metallic plate 2 is first folded to have a first folded portion 21 along a first edge by means of the press with the help of both the notch 31 of the lower die 3 and the sharp lower end of the upper die 4, which first folded portion 21 defines a right angle, as presented with the second drawing in FIG. 4. Next, the metallic plate 2 is curved to have a curved portion 22 next to the first folded portion 21 with the help of the shallow trench 51 and the pressing portion of the upper die 6, as presented with the third and the fourth drawings in FIG. 4; the curved portion 22 has such a shape that the two flat portions adjacent to it aren't parallel with each other, as presented with the fifth drawing in FIG. 4. Then, the curved portion 22 of the plate 2 is sandwiched between and pressed with both the deeper trench 52 of the lower die 5 and the pressing portion of the upper die 6 such that the two straight portions adjacent to it become nearly parallel with each other, as presented with the fifth and the sixth drawings in FIG. 4. And, the metallic plate 2 is folded to have right angles along two ones of the edges that are adjacent to the curved portion 22 also by means of the press with the help of both the notch 31 of the lower die 3 and the sharp lower end of the upper die 4.
The above method is found to have the following disadvantages:    1. The metallic plate has resilience therefore it is difficult to make the metallic plate change to and remain in such a shape that two straight portions thereof that are adjacent to the curved portion are parallel with each other.    2. Because the metallic plate has resilience, the deeper trench 52 of the lower die 5 has to be used otherwise the metallic plate 2 can't be formed with a curved portion having such a shape that the two opposing straight portions of the plate 2 are nearly parallel with each other. In addition, the metallic plate 2 is prone to get damaged if it is formed with the curved portion directly by means of the deeper trench 52 of the lower die 5 without using the shallower trench 51 before. Therefore, the two trenches 51, 52 have to be used in two consecutive pressing actions, and in turns, the worker will face the difficulty of placing the curved portion 22 in the correct position relative to the deeper trench 52 after the curved portion 22 has been formed with the help of the shallower trench 51.    3. Because the two opposing straight portions of the metallic plate 2 on two sides of the curved portion 22 are not exactly parallel with each other, the worker has to press the straight portions manually while he is connecting the edges of the metallic plate by means of welding, facing much trouble. Consequently, it takes more labor to make the metallic plate become a finished product, i.e. a desk.